feat(roaming): add Roaming page with 3x3 Maidenhead grid

Ports the QTH定位器 (com.us1pm.gridsquarelocator) location panel into Look4Sat as a new 'Roaming' page, restyled with the app's own look.

UI (top to bottom):
- Info header: GPS status dot, big 8-char locator, Lat/Lon rows with DMS + 5-decimal display, and a GPS 定位 button
- 3x3 grid panel: the current 4-char Maidenhead square (e.g. OL42) centered, surrounded by its 8 neighbors (OL33..OL51), with a red position marker in the center cell

Logic:
- QthConverter gains qthNeighbors(square) building the 3x3 grid with field/square carry at boundaries (AA00 wraps to RR99, IO91 crosses into J field), and qthToSquare(locator) extracting the 4-char square
- Verified against the decompiled app algorithm and the reference screenshot (OL42 grid matches exactly); 9 unit tests cover normal, boundary and field-wrap cases

Navigation:
- New bottom-nav item 'Roaming' between Match and Settings, with a crosshair icon
- New feature:roaming module (ViewModel + Compose screen) registered in the app

Build config:
- Lowered Gradle JVM heap from -Xmx6g to 768m: the 2GB build server froze on the old value; heavy release builds stay on GitHub Actions

feature/roaming/src/main/java/com/rtbishop/look4sat/feature/roaming/RoamingScreen.kt

@@ -0,0 +1,216 @@
+/*
+ * Look4Sat. Amateur radio satellite tracker and pass predictor.
+ * Copyright (C) 2019-2026 Arty Bishop and contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.rtbishop.look4sat.feature.roaming
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.aspectRatio
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.shape.CircleShape
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.text.font.FontFamily
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import com.rtbishop.look4sat.core.presentation.CardButton
+
+private val InfoCyan = Color(0xFF0CCFE2)
+private val GridBlue = Color(0xFF12ACE0)
+private val LabelBlue = Color(0xFF135678)
+private val PositionRed = Color(0xFFFF0000)
+private val DividerWhite = Color(0xFFF7F7F7)
+
+@Composable
+fun RoamingScreen(viewModel: RoamingViewModel) {
+    val state by viewModel.uiState.collectAsStateWithLifecycle()
+    Column(modifier = Modifier.fillMaxSize()) {
+        InfoHeader(state = state, onGpsClick = viewModel::onGpsClick)
+        GridPanel(state = state)
+    }
+}
+
+@Composable
+private fun InfoHeader(state: RoamingState, onGpsClick: () -> Unit) {
+    Column(
+        modifier = Modifier
+            .fillMaxWidth()
+            .background(InfoCyan)
+            .padding(horizontal = 16.dp, vertical = 10.dp)
+    ) {
+        // Top row: GPS status + big locator
+        Row(verticalAlignment = Alignment.CenterVertically) {
+            Text(
+                text = "GPS",
+                fontSize = 20.sp,
+                fontWeight = FontWeight.Medium,
+                color = Color.Black
+            )
+            Spacer(modifier = Modifier.width(8.dp))
+            Box(
+                modifier = Modifier
+                    .size(12.dp)
+                    .clip(CircleShape)
+                    .background(if (state.qthLocator != "----") Color(0xFF3BAF52) else Color.Gray)
+            )
+            Spacer(modifier = Modifier.weight(1f))
+            Text(
+                text = state.qthLocator,
+                fontSize = 30.sp,
+                fontWeight = FontWeight.Bold,
+                fontFamily = FontFamily.Monospace,
+                color = Color.Black
+            )
+        }
+        Spacer(modifier = Modifier.height(10.dp))
+        // Lat / Lon rows with DMS + decimal
+        CoordinateRow(label = "Lat", value = state.latitude, isLat = true)
+        Spacer(modifier = Modifier.height(4.dp))
+        CoordinateRow(label = "Lon", value = state.longitude, isLat = false)
+        Spacer(modifier = Modifier.height(10.dp))
+        CardButton(
+            onClick = onGpsClick,
+            text = "GPS 定位",
+            modifier = Modifier.fillMaxWidth()
+        )
+    }
+}
+
+@Composable
+private fun CoordinateRow(label: String, value: Double, isLat: Boolean) {
+    Row(
+        verticalAlignment = Alignment.CenterVertically,
+        modifier = Modifier.fillMaxWidth()
+    ) {
+        Text(
+            text = label,
+            fontSize = 16.sp,
+            fontWeight = FontWeight.Medium,
+            color = Color.Black,
+            modifier = Modifier.width(44.dp)
+        )
+        Text(
+            text = formatDms(value, isLat),
+            fontSize = 15.sp,
+            color = Color.Black
+        )
+        Spacer(modifier = Modifier.weight(1f))
+        Text(
+            text = "${formatDecimal(value)}°",
+            fontSize = 14.sp,
+            color = LabelBlue
+        )
+    }
+}
+
+@Composable
+private fun GridPanel(state: RoamingState) {
+    Column(
+        modifier = Modifier
+            .fillMaxSize()
+            .background(GridBlue)
+            .padding(4.dp)
+    ) {
+        // 3x3 grid: row 0 = north
+        for (row in 0..2) {
+            Row(
+                modifier = Modifier
+                    .fillMaxWidth()
+                    .weight(1f),
+                horizontalArrangement = Arrangement.spacedBy(4.dp)
+            ) {
+                for (col in 0..2) {
+                    val index = row * 3 + col
+                    val isCenter = index == 4
+                    GridCell(
+                        label = state.gridSquares.getOrElse(index) { "----" },
+                        isCenter = isCenter,
+                        showMarker = isCenter && state.qthLocator != "----",
+                        modifier = Modifier.weight(1f)
+                    )
+                }
+            }
+            if (row < 2) Spacer(modifier = Modifier.height(4.dp))
+        }
+    }
+}
+
+@Composable
+private fun GridCell(
+    label: String,
+    isCenter: Boolean,
+    showMarker: Boolean,
+    modifier: Modifier = Modifier
+) {
+    Box(
+        contentAlignment = Alignment.Center,
+        modifier = modifier
+            .aspectRatio(1f)
+            .background(if (isCenter) InfoCyan else GridBlue)
+            .border(1.dp, DividerWhite)
+    ) {
+        Column(horizontalAlignment = Alignment.CenterHorizontally) {
+            Text(
+                text = label,
+                fontSize = if (isCenter) 26.sp else 16.sp,
+                fontWeight = if (isCenter) FontWeight.Bold else FontWeight.Normal,
+                color = LabelBlue,
+                textAlign = TextAlign.Center
+            )
+            if (showMarker) {
+                Spacer(modifier = Modifier.height(4.dp))
+                Box(
+                    modifier = Modifier
+                        .size(12.dp)
+                        .background(PositionRed)
+                        .border(2.dp, DividerWhite)
+                )
+            }
+        }
+    }
+}
+
+private fun formatDms(value: Double, isLat: Boolean): String {
+    val abs = kotlin.math.abs(value)
+    val deg = abs.toInt()
+    val minFull = (abs - deg) * 60
+    val min = minFull.toInt()
+    val sec = ((minFull - min) * 60).toInt()
+    val hemi = if (value >= 0) (if (isLat) "N" else "E") else (if (isLat) "S" else "W")
+    return "$deg° $min' $sec\" $hemi"
+}
+
+private fun formatDecimal(value: Double): String = String.format("%.5f", value)

feature/roaming/src/main/java/com/rtbishop/look4sat/feature/roaming/RoamingViewModel.kt

@@ -0,0 +1,83 @@
+/*
+ * Look4Sat. Amateur radio satellite tracker and pass predictor.
+ * Copyright (C) 2019-2026 Arty Bishop and contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.rtbishop.look4sat.feature.roaming
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import androidx.lifecycle.viewmodel.initializer
+import androidx.lifecycle.viewmodel.viewModelFactory
+import com.rtbishop.look4sat.core.domain.predict.GeoPos
+import com.rtbishop.look4sat.core.domain.repository.IMainContainer
+import com.rtbishop.look4sat.core.domain.repository.ISettingsRepo
+import com.rtbishop.look4sat.core.domain.utility.positionToQth
+import com.rtbishop.look4sat.core.domain.utility.qthNeighbors
+import com.rtbishop.look4sat.core.domain.utility.qthToSquare
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.update
+import kotlinx.coroutines.launch
+
+data class RoamingState(
+    val latitude: Double = 0.0,
+    val longitude: Double = 0.0,
+    val altitude: Double = 0.0,
+    val qthLocator: String = "----",
+    val gridSquares: List<String> = List(9) { "----" },
+    val isUpdating: Boolean = false,
+    val messageResId: Int = 0
+)
+
+class RoamingViewModel(private val settingsRepo: ISettingsRepo) : ViewModel() {
+
+    private val _uiState = MutableStateFlow(RoamingState())
+    val uiState: StateFlow<RoamingState> = _uiState
+
+    init {
+        viewModelScope.launch {
+            settingsRepo.stationPosition.collect { geoPos ->
+                _uiState.update { it.copy(isUpdating = false, messageResId = 0).withPosition(geoPos) }
+            }
+        }
+    }
+
+    fun onGpsClick() {
+        val success = settingsRepo.setStationPosition()
+        if (success) {
+            _uiState.update { it.copy(isUpdating = true) }
+        }
+    }
+
+    private fun RoamingState.withPosition(pos: GeoPos): RoamingState {
+        val locator = positionToQth(pos.latitude, pos.longitude) ?: return copy(qthLocator = "----")
+        val square = qthToSquare(locator)
+        val squares = if (square != "----") qthNeighbors(square) else List(9) { "----" }
+        return copy(
+            latitude = pos.latitude,
+            longitude = pos.longitude,
+            altitude = pos.altitude,
+            qthLocator = locator,
+            gridSquares = squares
+        )
+    }
+
+    companion object {
+        fun factory(container: IMainContainer) = viewModelFactory {
+            initializer { RoamingViewModel(container.settingsRepo) }
+        }
+    }
+}